The present invention relates to a process designed to increase the resistance of bacterial strains, in particular Thiobocillus ferrooxidans, to As(III) and As(V), and also the bacteria thereby obtained and their application in particular in the extraction of gold from arsenopyrite deposits.
Thiobacillus ferrooxidans bacteria are the main microorganisms which leach metal sulfides: pyrites and CuS, for example, and also uranium ores. They are autotrophic bacteria when they grow in the presence of Fe.sup.2+ ions and heterotrophic when they grow on an organic medium (for example glucose). There are already a few industrial installations which carry out this technique of bacterial leaching, chiefly for copper (USA) and uranium (South Africa, Canada).
The industrial techniques for leaching uranium have as their object the conversion of the uranium, which is present in ores in the form of insoluble oxide, to water-soluble uranium salt. The uranium core frequently contains pyrite (FeS.sub.2) and the action of Thiobacillus ferrooxidans takes the effect in the following manner: EQU FeS.sub.2 +3/2O.sub.2 +H.sub.2 O.fwdarw.FeSO.sub.4 +H.sub.2 SO.sub.4 ( 1) ##STR1## EQU and UO.sub.2 +Fe.sub.2 (SO.sub.4).sub.3 .fwdarw.UO.sub.2 SO.sub.4 +2FeSO.sub.4 ( 3)
UO.sub.2 SO.sub.4 is soluble in water.
Since the oxidation of U(IV) to U(IV) is faster in the presence of the bacteria than in the presence of Fe.sup.3+ ions alone, recent studies suggest that there is a direct action of Thiobacillus ferrooxidans on U(IV): EQU 2UO.sub.2 +O.sub.2 +2H.sub.2 SO.sub.4 .fwdarw.2UO.sub.2 SO.sub.4 +2H.sub.2 O (4)
Using the oxidizing capacities of this bacterium, 85% of the uranium in a low-grade ore (200 to 300 ppm of uranium) can be leached in 20 weeks. If the ore is more finely ground (particle size: 0.8 to 1.2 mm), in 9 weeks 60 to 70% of the uranium can be leached by the bacteria.
The reaction (2) is the basis of the process for bacterial leaching of uranium.
Thiobacillus ferrooxidans can also be used in order to extract gold from arsenopyrite deposits. The autotrophic bacterium digests the arsenopyrite and enables the metal to be extracted without having to roast the ore.
Unfortunately, the majority of strains of Thiobacillus ferrooxidans are very sensitive to As(III) and As(V) toxicity.